Add plugins input to GitHub Action

This commit adds support for installing Claude Code plugins via a new `plugins` input parameter.

Changes:
- Added `plugins` input to action.yml (comma-separated list)
- Created `install-plugins.ts` with plugin installation logic
- Added comprehensive tests in `install-plugins.test.ts`
- Updated base-action index.ts to call plugin installation
- Plugins are installed after settings setup but before Claude execution

Usage example:
```yaml
- uses: anthropic-ai/claude-code-action@main
  with:
    plugins: "feature-dev,test-coverage-reviewer"
```

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

base-action/src/install-plugins.ts

@@ -0,0 +1,80 @@
+#!/usr/bin/env bun
+
+import { spawn } from "child_process";
+
+// Declare console as global for TypeScript
+declare const console: {
+  log: (message: string) => void;
+  error: (message: string) => void;
+};
+
+/**
+ * Parses a comma-separated list of plugin names and returns an array of trimmed plugin names
+ */
+export function parsePlugins(pluginsInput: string | undefined): string[] {
+  if (!pluginsInput || pluginsInput.trim() === "") {
+    return [];
+  }
+
+  return pluginsInput
+    .split(",")
+    .map((plugin) => plugin.trim())
+    .filter((plugin) => plugin.length > 0);
+}
+
+/**
+ * Installs a single Claude Code plugin
+ */
+export async function installPlugin(
+  pluginName: string,
+  claudeExecutable: string = "claude",
+): Promise<void> {
+  return new Promise((resolve, reject) => {
+    const process = spawn(claudeExecutable, ["plugin", "install", pluginName], {
+      stdio: "inherit",
+    });
+
+    process.on("close", (code: number | null) => {
+      if (code === 0) {
+        resolve();
+      } else {
+        reject(
+          new Error(
+            `Failed to install plugin '${pluginName}' (exit code: ${code})`,
+          ),
+        );
+      }
+    });
+
+    process.on("error", (err: Error) => {
+      reject(
+        new Error(`Failed to install plugin '${pluginName}': ${err.message}`),
+      );
+    });
+  });
+}
+
+/**
+ * Installs Claude Code plugins from a comma-separated list
+ */
+export async function installPlugins(
+  pluginsInput: string | undefined,
+  claudeExecutable: string = "claude",
+): Promise<void> {
+  const plugins = parsePlugins(pluginsInput);
+
+  if (plugins.length === 0) {
+    console.log("No plugins to install");
+    return;
+  }
+
+  console.log(`Installing ${plugins.length} plugin(s)...`);
+
+  for (const plugin of plugins) {
+    console.log(`Installing plugin: ${plugin}`);
+    await installPlugin(plugin, claudeExecutable);
+    console.log(`✓ Successfully installed: ${plugin}`);
+  }
+
+  console.log("All plugins installed successfully");
+}